Conventional measurement converters for measuring electronic power are based on the principle of the electromechanical conversion of the product of the voltage and the current into a mechanical moment of force. It is regarded to be a disadvantage of this electromechanical conversion that the conversion is possible only within a narrow frequency range and that the conversion accuracy is only slight.
Advances in the field of electronics led to electronic measurement converters. Known electronic measurement converters for the precision measurement of electric power generally consist of circuits, which convert the instantaneous values of the input signals into pulse or digital signals for the subsequent processing. The disadvantage of these circuits consists therein that, because of the scanning principle, the accuracy of measurement at higher frequencies is limited by the scanning frequency. The conventional semi-conductor elements limit the maximum frequency of the measurement signals to values between 10.sup.3 and 10.sup.5 Hz, depending on the measurement accuracy required. The conversion error attained hereby is about 10.sup.-4 (Seppa et al., "A Digital Three-Phase Watthour Meter", Conference on Precision Electromagnetic Measurements, Conference Digest, Ottawa, June 1990).
Interference between the measurement frequency and the scanning frequency, as well as the problematic processing of pulse signals are also regarded as disadvantages of equipment using the scanning principle.
Multiplier equipment, based on non-linear circuits or on the Hall effect, admittedly is structurally simple; however, a higher accuracy of measurement and a long-term stability are not possible with such equipment. Circuits, based on square characteristics of the diodes (U.S. Pat. No. 5,266,888), can be used only with reduced measurement accuracy to approximate the effective values of the individual input signals.
The most accurate methods for measuring electric power are based on thermal effects. For these methods, thermoelectric converters are used. Conventional thermoelectric converters can convert only the effective value of a signal, such as voltage, current, TEM wave (DE 35 39 402 C2, DE 34 28 077 A1) or beam power (DE 42 43 597 C2, DE 34 08 724 C2). Also known are thermoelectric converters, which utilize the temperature-dependent resistance of thermistors for measuring the effective value of the voltage (U.S. Pat. No. 5,189,362). Power, which is determined by two independent signals, can be measured only indirectly with the help of these conventional thermoelectric converters, by the effective values of the linear combinations of input signals. In the best cases, the error of measurement is about 10.sup.-5 here. The power is measured indirectly by the effective values of linear combinations of input signals. The measurement error is about 10.sup.-5 here. These thermoelectric converters, however, have a complicated construction and therefore are expensive. (K. Takahashi, S. Kusui, "A Precision Thermal Wattmeter Having a Sampling Control System and an Analog Feedback Loop", Conference on Precision Electromagnetic Measurements, Conference Digest, Ottawa, June 1990).
Thermal multiplication by means of a controlled resistance and by means of a temperature sensor, both of which are disposed on a common support insulated from the environment (DE 4117133 C1) is also known. The sensitivity to the direct current portion of the signals to be measured and the effect of lead wire resistances on the accuracy of measurement are disadvantages of this simple arrangement.
For the conventional thermal insulation of load resistances of the thermoelectric measurement converter, a thin dielectric membrane, consisting of several layers, is used (M. Klonz, "Current Developments in Accurate AC-DC Difference Measurements", Conference Digest, Boulder, Colo., USA, 1994). In the case of a different, known solution for thermal insulation, an island, produced by etching, is provided within a substrate and only the solid, metal wire leads to the electronic components are mechanically fixed on this island (U.S. Pat. No. 4,257,061 and L. Harold Slott, "A Multirange Standard for AC/DC Difference Measurements", IEEE Trans. on Inst. and Meas. Im-35, No. 4, Dec. 1986). These lead wires must be produced from a material, that has a high ratio of electrical conductivity to thermal conductivity. The disadvantage of the simple membrane is the problematic mechanical stability to stresses, which are caused by temperature gradients in the membrane. The production of multilayered membranes is technologically complicated and therefore expensive. The complicated and, with that, expensive manufacturing process is a disadvantage of the construction, in which the lead wires are fixed to the free island.